Copper and copper alloys have been used for millennia as some of mankind's primary technological materials. Their combination of ease of manufacture, recyclability, resistance to overall corrosion, and their availability on a variety of attractive colors and finishes have made them the preferred material for coinage, as well as a variety of artistic and architectural applications where these properties are important. Electrical and thermal conductivity greater than nearly all competitive materials combined with useful strength, formability, and relatively low cost have made these materials vital to the electronics industry.
Copper is an essential trace mineral, vital to the health and proper functioning of human metabolism, as well as other life forms at very low concentrations.
Copper sheathing of ships' hulls was used by the British Navy beginning in the 18th century to prevent attack by teredo (shipworm) and to prevent attachment of marine weeds and organisms such as barnacles to wooden-hulled ships. The beneficial effects were due to slow dissolution of the copper surface in contact with seawater. Also, copper and copper compounds have been used in paints for ships' hulls made of a variety of materials for their effectiveness in preventing fouling of ships' bottoms by marine organisms. These antifouling properties are tied to the release of copper ions from the affected surface, resulting in a microenvironment at the surface which is toxic to such organisms and preventing attachment of these organisms to the affected surface. Marine microorganisms may be affected by as little as 1 part per billion copper (1 ppb Cu).
Recent studies have shown that copper alloy surfaces are effective at decreasing the viability of microorganisms such as salmonella, listeria, and E. coli which cause food-borne illnesses. Such surfaces are also effective at reducing viability of microorganisms tied to secondary infections in health care facilities, such as staphylococcus aureus, legionella, and others.
Traditionally, copper alloy products are produced with a bright surface protected from oxidation by a variety of treatments. Copper and copper alloys will naturally form a thin oxide layer in contact with the atmosphere, consisting primarily of cuprous oxide (Cu2O) at normal temperatures; in environments containing sulfur, there is an increased proportion of cupric oxide (CuO) and cupric sulfide (CuS). This layer will grow thicker over time, eventually obscuring the bright surface and causing the surface to darken. Dark films of oxides and/or sulfides on the surface are considered “dirty” and objectionable, unless used deliberately for specific decorative or architectural purposes. A great deal of effort and research has gone into methods of preventing such films from forming and of removing them when they do form. Application of surface treatments (anti-tarnish films, stain inhibitors, or polymer coatings) which slow the transport of oxygen to the copper alloy surface also slows formation of oxide films. These and other methods are well known to those skilled in the art.
Since the antimicrobial properties of copper, copper alloys and copper compounds have been known for some time, there have been a number of patents issued for materials and processes making use of these properties. As noted above, copper sheathing has been used for centuries to prevent biofouling of ship hulls; more recently, static underwater structures such as oil platforms have been similarly protected. Galvanic corrosion between the steel of the platforms and the protective copper sheathing has limited the usefulness of this method, but Miller (U.S. Pat. No. 4,987,036; 1/1991) discloses a method of creating a substantially continuous coating by placement of numerous small platelets of copper adhered to the structure with an electrically insulating material. Inoue (U.S. Pat. No. 5,338,319; 2/1995) discloses a related method for coating the inside of a resin pipe with a beryllium-containing copper alloy. Both methods involve contact with seawater.
Another patent (Miyafuji U.S. Pat. No. 6,313,064; 11/2001) makes use of a Cu—Ti alloy where the titanium (and possibly other alloying elements) preferentially oxidizes. Although this does rely on a deliberate surface treatment to produce oxides and available ions at the metal surface, these oxides and ions include other and more reactive elements than just copper sulfides and oxides and copper ions.
Many patents have been issued for copper-containing biocides for use on agricultural produce and in water treatment. Copper salts and compounds provide a strong source of antimicrobially effective copper ions, but the relatively high solubility of the compounds results in short periods of effectiveness before the copper is washed away. Many of the patents focus on methods to decrease the release of copper into solution and increase the effective lifetime of the treatment. Examples of this type of product are given in Cook (U.S. Pat. No. 7,163,709; 1/2007), Back (U.S. Pat. No. 6,638,431; 10/2003), Stainer (U.S. Pat. No. 5,171,350; 12/1992), and Denkewicz (U.S. Pat. No. 6,217,780; 4/2001). These treatments may be applied to a variety of surfaces, but they do not make use of a permanent, inherently antimicrobial copper or copper alloy surface to act as a long-term source of copper ions.
Another method used to make metallic mill products (such as metal sheet or strip in coils) with an antimicrobial surface is to coat the surface with a solution, paint, or polymer containing an antimicrobial agent and dry or cure the coating in place. The antimicrobial agent may be metallic particles, non-metallic particles carrying antimicrobial metal ions, glass particles containing such ions, and/or particles of metal salts or similar compounds. The classic example of these methods is the “HealthShield” product line from AK Steel (Myers, et al.; U.S. Pat. No. 6,929,705; 8/2005), consisting of a metallic substrate coated with a resin formulation carrying inorganic zeolites and oxides which in turn carry metal ions or compounds for antimicrobial effect. Other similar products (directly using metal compounds or salts) are disclosed in Lyon (U.S. Pat. No. 6,042,877; 3/2000) and Zlotnik (U.S. Pat. No. 5,066,328; 11/1991), although this list is by no means exhaustive. While these coatings may be applied to a number of different substrates, either before or after fabrication into finished articles, the antimicrobial properties of these items are due to the coating alone and do not rely on the metallic article itself as a permanent source of antimicrobial ions.
Yet another method of forming antimicrobial articles and surfaces also involves the use of particles of metal powders, metal-ion containing salts and other compounds, and metal-ion carrying particles similar to those noted above, but blended throughout a bulk polymer or similar moldable substance. McDonald (U.S. Pat. No. 6,797,743; 9/2004) discloses such a polymer, also used as a coating on a substrate item; Kiik (U.S. Pat. No. 6,585,813; 7/2003) discloses a related formulation used to fight algae growth on blended asphalt roofing shingles and other items used in the building trades. Again, the anti-microbial properties are due to the copper- or other metal-containing particles, and not due to the bulk of the material itself. Also, the effectiveness of these materials is limited by the total concentration of anti-microbial metal particles and compounds which can be blended into the matrix, and by transport of these effective ions through the matrix to the useful surface, where an uncoated metal surface presents the effective ions directly at the surface with minimal transport and concentration limited only by the solubility of the metal in the solution of interest.
One disadvantage of the traditional method of supplying copper surfaces free of oxidation and treated to prevent further oxidation is that a clean, bare, bright copper surface is generally hydrophobic, minimizing or preventing contact between the surface and water or aqueous solutions. Treatments normally applied to prevent further oxidation are generally even more hydrophobic than the original copper surface, both directly minimizing physical transport of oxygen to the copper surface and preventing formation of adsorbed films of water on the surface which can assist transport of oxygen to the surface and copper ions from the surface.
A further disadvantage of such treatments is that clean, bare, bright copper in the metallic, non-ionized state is nearly insoluble in water. Oxidation of copper provides copper ions which can be assimilated into aqueous solutions or into body fluid residues to provide antimicrobial properties. Without such copper ions available for transport, an antimicrobially active surface would need to develop naturally. Not only can these natural/atmospheric processes be slow to occur, but the reactions required are variable in reaction time, dependent on the nature of prior commercial treatment, environmental conditions, and, therefore, are difficult to predict. One interested in ensuring that a surface is active at the time it is placed in service would benefit from the stated invention(s), as they ensure the surface is predictably active at the time it is placed in service. It is, therefore, difficult to predict the antimicrobial activity of these naturally formed surfaces.
Prior art does not address the effects of manufacturing methods necessary to create commercially useful articles and how those stated antimicrobial surfaces could be changed in processing. The invention is directed to the problem of creating a repeatably and renewably active surface at the time an article is placed in service which provides copper ions available for assimilation into aqueous solutions or body fluid residues for antimicrobial properties, which can be produced on semi-finished goods or finished articles during or after manufacture.